1. Field of the Invention
This invention relates to a semiconductor device and its fabrication, and more particularly to a junction-free NAND flash memory and a fabricating method thereof.
2. Description of Related Art
Flash memory is a type of non-volatile memory that can be programmed and erased electrically and can retain data even after the power is off, which is widely adopted in personal computers (PC) and other electronic apparatuses.
A conventional method for fabricating a MOS memory cell includes, after the formation of a stacked gate structure constituted of a tunnel oxide layer, a floating gate, a dielectric layer and a control gate, performing a doping step to the substrate with the gate structure as a mask. With a long period of high-temperature annealing conducted subsequently, source/drain regions with a larger area and a deeper junction is formed.
In programming of the memory, appropriate biases depending upon the channel length are applied to the source/drain regions from corresponding bit lines, such that hot electrons are produced in the channel and partially injected to the floating gate through the tunnel oxide.
In the trend of high device integration, the device size is reduced so that the channel length of the floating gate is reduced, while the depletion regions produced by the source/drain regions further reduce the channel length. Since the annealing of the source/drain is conducted at a high temperature for a long period, the depletion region of the source may merge with that of the drain. This makes problems such as short channel effect and punch-through leakage worse.